Curbs on emissions of carbon dioxide and other substances have been strengthened against a background of a growing environmental protection movement, and in the automobile world there is now vigorous development of electric vehicles (EVs) and hybrid electric vehicles (HEVs) alongside vehicles using fossil fuels such as gasoline, diesel oil and natural gas. In addition, the soaring price of fossil fuels in recent years has acted to spur on the development of EVs, HEVs and the like.
The batteries used for such EVs, HEVs and the like are generally nickel-hydrogen secondary batteries or lithium ion secondary batteries. However, what is now required of EVs and HEVs is not only environmental friendliness, but also basic performance as an automobile, in other words, superior driving performance. Therefore, it is necessary not simply to enlarge the capacity, but also to increase the output, of the batteries used for EVs and HEVs, in order to effect large improvements in these vehicles' acceleration and hill-climbing performance. However, when a high output is discharged, a large current flows in the battery, and as a result there is a large increase in heat due to contact resistance between the substrates and the collectors, which are the generation elements. Thus, batteries for EVs and HEVs are required not only to be large-sized and large capacity, but also to handle a large current. Accordingly, with the object of preventing electricity loss inside the battery and thereby reducing heat emission, many improvements have been carried out with regard to lowering the internal resistance by preventing welding faults between the substrates and collectors, which are the generation elements.
There exist the methods of mechanical caulking, welding and the like for electrically joining the substrates and collectors, which are the generation elements. Welding is appropriate as the electrical collection method for batteries of which high output is required. Also, in a lithium ion secondary battery, in order to effect low resistance, the material used for the positive electrode substrates and collector is aluminum or aluminum alloy, and the material used for the negative electrode substrates and collector is copper or copper alloy. However, these materials have the characteristics of low electrical resistance and high thermal conductivity, so that an extremely large amount of energy is required in order to weld them.
The following methods have long been known as methods for welding together the substrates and collectors which are the generation elements:
1) Laser welding (see JP-A-2001-160387)
2) Ultrasonic welding (see JP-A-2007-053002)
3) Resistance welding (see JP-A-2006-310254)
With the laser welding method, a high-energy laser beam is required because the reflectivity of the aluminum, aluminum alloy, copper, or copper alloy welded material with respect to the YAG (yttrium-aluminum garnet) laser light that is widely used to weld metals is high—around 90%. There also exist the problems that when aluminum, aluminum alloy, copper, or copper alloy is laser-welded, the weldability varies greatly depending on the condition of the surfaces, and that the occurrence of spattering is unavoidable, as in laser welding of other materials.
Ultrasonic welding also requires a large amount of energy, because the thermal conductivity of the aluminum, aluminum alloy, copper, or copper alloy welded material is high. Also, the positive electrode active material and/or negative electrode active material may be dislodged by the ultrasonic vibration during welding. Accordingly, in the invention disclosed in JP-A-2007-053002, the electrode assembly, which is the generation element, is compressed during ultrasonic welding, so that dislodged negative electrode active material will not enter inside it.
Further, with resistance welding, due to the aluminum, aluminum alloy, copper, or copper alloy welded material having low electrical resistance and high thermal conductivity there exist the problems that large current needs to be input in a short time, that fusion-joining of the collectors and the electrode rods which are used in resistance welding sometimes occurs during welding, and that melting or spark generation may occur at places other than the welds.
Thus, the three welding methods have their merits and drawbacks. In the interests of productivity and economy however, the resistance welding method, which has long been used as a method for welding between metals, will preferably be employed. With the electrode assembly (see JP-A-2002-008708) of EV and HEV application sealed batteries however, since the exposed portions of the positive electrode substrates and negative electrode substrates have a large number of stacked layers, a great deal of welding energy is necessary in order to firmly resistance-weld the collector made of aluminum or aluminum alloy to the positive electrode substrates, and the collector made of copper or copper alloy to the negative electrode substrates. Moreover, when the welding energy is rendered large for resistance welding, the generation of spattered particles is increased and there is increased probability that the particles will move into the inside of the electrode assembly, so that an internal short circuit is caused.
JP-A-2007-287597 discloses the invention of a storage battery in which the electrode substrates are resistance-welded to the peripheral wall of a hole (burring hole) provided in the collectors. However, with the invention disclosed in JP-A-2007-287597, the collectors are welded to the substrates while being pressed, in a perpendicular direction, against the forward edge portions of the substrates, so that the collectors are not in surface contact with the substrates, and consequently it is not possible to, control the dispersion direction of the spattered particles. Also, with the invention disclosed in JP-A-2007-287597, the number of substrates that contact with the rim portion of the hole provided in the collectors is small (one substrate), and therefore the quantity of molten metal (nuggets) occurring during the resistance welding is small, so that the spattered particles cannot be captured.